I’ve often said, “Bad concrete has been very good to me.” As a technical specialist in the commercial resilient flooring industry, concrete issues are an almost daily topic of discussion, and my clients need answers.
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Flooring failures attributed to moisture-related concrete problems are at near epidemic proportions today. Coatings, carpet, vinyl, rubber, wood, laminates and most floor coverings are affected to one degree or another by pH issues and excess water vapor emissions through a concrete slab. Moisture causes gaps between tiles, adhesive oozing, bumps, dents, cupping, bubbles, indentations, wheel marks and more. Left uncorrected, these problems can evolve into health and safety issues caused by mold, mildew and floors lifting. Here’s a crash course on concrete floors and the issues related to flooring installed over concrete. I hope it will go a long way to helping you to understand what could go wrong.
First, using the right terminology is helpful because words like “cement” and “curing” are often misused.
“Cement” isn’t the finished product, but is the grey powdery ingredient in concrete. Cement powder, water, sand and aggregate (rocks) bond together to form concrete. There’s no such thing as “a cement floor.” Another common misunderstanding is that “cured” means the concrete is ready for floor coverings.
The term “28-day cure” is the approximate time a 4-in. slab takes to cure, and is often used as a guideline for when to install a floor. However, after 28 days, the concrete isn’t dry because curing and drying aren’t the same things. Curing is the chemical reaction that bonds those ingredients together to make concrete. Drying refers to evaporating the excess water (about 2/3 of the water in the mix) from the concrete after the curing process is complete.
The industry standard, ASTM F 710, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring, describes the process and uses the terms cure and dry in the same sentence - “New concrete slabs shall be properly cured and dried before installation of resilient flooring.”
How long to dry?
The drying time before slabs are ready for moisture testing depends on atmospheric conditions and mix design, according to F 710. A 4-in. thick slab allowed to dry from only one side typically requires 90 days to 120 days to achieve a moisture vapor emission rate (MVER) of 3 lb. water/1,000 sq. ft. per 24 hr (the resilient flooring industry standard MVER). Suspended slabs -– the second floor and above -- often contain a lot more water, and take even longer to dry. As noted in ASTM F 710, lightweight concrete, floors containing lightweight aggregate or excess water, and those that dry from only one side, such as concrete on metal deck construction, may need a much longer drying time. One laboratory study found that at ideal conditions (70°F, 50% relative humidity) it took 46 days for a standard mix to dry and 168 days for a lightweight mix. Those were both under ideal conditions – concrete with the right amount of water at the perfect temperature and humidity. In the real world it takes even longer.
Just as important as sealing your building’s roof is sealing the floor against moisture intrusion. ASTM F 710 says that every concrete floor slab on or below grade intended to receive resilient flooring should have a moisture retarder (often improperly called a vapor barrier) installed below the slab. The vapor retarder is often left out of the project to save money and speed the finishing of the concrete. Many failures on first-floor and lower-level slabs are caused by a missing or damaged vapor retarder.
Who’s responsible for failures?
The concrete contractor sometimes gets blamed. But if the concrete is mixed and placed according to the architect’s specifications, one can’t blame the contractor, unless extra water gets added to the truck or the vapor retarder gets damaged. Bad adhesive is rarely the cause because adhesive is almost never defective, but it can be attacked by moisture and the elevated pH levels that come with it. The floor maintenance team often gets the rap, but a floor must be under a lot of water for a long time for the adhesive to let go. Most often, in new buildings, the flooring is installed before the concrete has completely dried, the curing compound was never removed, or the climate control system isn’t yet operational. Then, when the building is occupied and the interior conditioned air is dry, the floor covering blocks the movement of excess moisture that migrates upward through the slab. In older buildings, a missing vapor retarder or external sources such as leaks, exterior grading or sprinklers can cause moisture to pass through a slab.